Circuit for an implantable device

ABSTRACT

An integrated circuit includes: a radio-frequency (RF) to direct current (DC) rectifying circuit coupled to one or more antenna on an implantable wirelessly powered device, the rectifying circuit configured to: rectify an input RF signal received at the one or more antennas and from an external controller through electric radiative coupling; and extract DC electric power and configuration data from the input RF signal; a logic control circuit connected to the rectifying circuit and a driving circuit, the logic control circuit configured to: generate a current for the driving circuit solely using the extracted DC electrical power; in accordance with the extracted configuration data, set polarity state information for each electrode; and a driving circuit coupled to one or more electrode, the driving circuit comprising current mirrors and being configured to: steer, to each electrode and via the current mirrors, a stimulating current solely from the generated current.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/383,646, filed Dec. 19, 2016, now allowed, which is a continuation ofU.S. application Ser. No. 14/796,067, filed Jul. 10, 2015, now U.S. Pat.No. 9,522,270, issued Dec. 20, 2016, which claims the benefit of U.S.Provisional Application Ser. No. 62/022,768, filed on Jul. 10, 2014. Allof these prior applications are incorporated by reference in theirentirety.

TECHNICAL FIELD

This application relates generally to implantable stimulators.

BACKGROUND

Modulation of excitable tissue in the body by electrical stimulation hasbecome an important type of therapy for patients with chronic disablingconditions, including chronic pain, problems of movement initiation andcontrol, involuntary movements, vascular insufficiency, heartarrhythmias and more. A variety of therapeutic intra-body electricalstimulation techniques can treat these conditions. For instance, devicesmay be used to deliver stimulatory signals to excitable tissue, recordvital signs, perform pacing or defibrillation operations, record actionpotential activity from targeted tissue, control drug release fromtime-release capsules or drug pump units, or interface with the auditorysystem to assist with hearing. Typically, such devices utilize asubcutaneous battery operated implantable pulse generator (IPG) toprovide power or other charge storage mechanisms.

SUMMARY

In one aspect, some implementations provide an integrated circuit for animplantable wirelessly powered device for implantation in a patient'sbody, the circuit including: a radio-frequency (RF) to direct current(DC) rectifying circuit coupled to one or more antenna on theimplantable wirelessly powered device, the rectifying circuit configuredto: rectify an input RF signal received at the one or more antennas andfrom an external controller through electric radiative coupling; andextract DC electric power and configuration data from the input RFsignal; a driving circuit coupled to one or more electrode andconfigured to: steer a stimulating current to each electrode accordingto the extracted configuration data to modulate the neural tissue withinthe patient's body; and a logic control circuit connected to therectifying circuit and the a driving circuit, the logic control circuitconfigured to: generate a current for the driving circuit solely usingthe extracted DC electrical power; in accordance with the extractedconfiguration data, set polarity state information for each electrode;and a driving circuit coupled to one or more electrode, the drivingcircuit comprising current mirrors and being configured to: steer, toeach electrode and via the current mirrors, a stimulating current solelyfrom the generated current to modulate neural tissue within thepatient's body.

Implementations may include one or more of the following features. Thedriving circuit may include: a current source digital to analog circuit(DAC) and a current sink digital to analog circuit (DAC), the currentsource DAC and the current sink DAC being complementary to each otherand both being mirrored connected to each electrode via the currentmirrors wherein the stimulating current may be determined by a currentmirror ratio and wherein the configuration data may include the currentmirror ratio. The current mirror ratio may equal matches the number ofelectrodes.

The driving circuit may include: a switch bank configured to control apolarity state for each electrode. The driving circuit may include: avariable shunt resistor adapted to reduce a ripple on the electrodeconnected thereto when the stimulating current has just ended.

The integrated circuit may further include: a discharge delay timer todelay the onset of capacitors coupled to each of the electrodes.

The integrated circuit may further include: a power on reset (PoR)circuit to maintain default state information for each electrode anddefault parameters for the stimulating current for each electrode, thePoR circuit configured to be trigged on by a rising edge and turned offby a falling edge.

The integrated circuit may further include: one or more address controlbits, configurable as a logic address of the implantable devicewirelessly powered by the extracted electric power. The integratedcircuit may further include a diode bridge.

The rectifying circuit may be coupled to a differential antenna on theimplantable wirelessly powered device. The rectifying circuit mayinclude: an amplitude modulation (AM) detection circuit to extractelectric power and configuration data from the input signal.

The logic control circuit may include: a state machine to record stateinformation for each electrode based on the extracted configurationdata, the configuration data including polarity setting for eachelectrode. The logic control circuit may be further configured to setpolarity state for each electrode during a communication initializationpulse. The logic control circuit may include a timer circuit to controla duration the generated current for the driving circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a high-level diagram of an example of a wirelessstimulation system.

FIG. 2 depicts a detailed diagram of an example of the wirelessstimulation system.

FIG. 3 is a flowchart showing an example of the operation of thewireless stimulation system.

FIG. 4 is a circuit diagram showing an example of a wireless implantablestimulator device.

FIG. 5 is a circuit diagram of another example of a wireless implantablestimulator device.

FIG. 6 is a block diagram showing an example of control and feedbackfunctions of a wireless implantable stimulator device.

FIG. 7 is a schematic showing an example of a wireless implantablestimulator device with components to implement control and feedbackfunctions.

FIG. 8 is a schematic of an example of a polarity routing switchnetwork.

FIG. 9A is a diagram of an example microwave field stimulator (MFS)operating along with a wireless implantable stimulator device.

FIG. 9B is a diagram of another example MFS operating along with awireless implantable stimulator device.

FIG. 10 is a detailed diagram of an example MFS.

FIG. 11 is a flowchart showing an example process in which the MFStransmits polarity setting information to the wireless implantablestimulator device.

FIG. 12 is another flow chart showing an example process in which theMFS receives and processes the telemetry feedback signal to makeadjustments to subsequent transmissions.

FIG. 13 is a schematic of an example implementation of power, signal andcontrol flow on the wireless implantable stimulator device.

FIG. 14 is a diagram of an example application-specific integratedcircuit (ASIC) chip for implantable use.

FIG. 15 shows an example sequence during operation of the ASIC chipshown in FIG. 14.

FIG. 16A shows an example current steering feature for the ASIC chipshown in FIG. 14.

FIG. 16B shows example waveforms simulated based on the depicted ASICchip model.

FIG. 17 shows example waveforms at various points in an ASIC chip withthe current steering feature.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In various implementations, systems and methods are disclosed forapplying one or more electrical impulses to targeted excitable tissue,such as nerves, for treating chronic pain, inflammation, arthritis,sleep apnea, seizures, incontinence, pain associated with cancer,incontinence, problems of movement initiation and control, involuntarymovements, vascular insufficiency, heart arrhythmias, obesity, diabetes,craniofacial pain, such as migraines or cluster headaches, and otherdisorders. In certain embodiments, a device may be used to sendelectrical energy to targeted nerve tissue by using remote radiofrequency (RF) energy without cables or inductive coupling to power apassive implanted wireless stimulator device. The targeted nerves caninclude, but are not limited to, the spinal cord and surrounding areas,including the dorsal horn, dorsal root ganglion, the exiting nerveroots, nerve ganglions, the dorsal column fibers and the peripheralnerve bundles leaving the dorsal column and brain, such as the vagus,occipital, trigeminal, hypoglossal, sacral, coccygeal nerves and thelike.

A wireless stimulation system can include an implantable stimulatordevice with one or more electrodes and one or more conductive antennas(for example, dipole or patch antennas), and internal circuitry forfrequency waveform and electrical energy rectification. The system mayfurther comprise an external controller and antenna for transmittingradio frequency or microwave energy from an external source to theimplantable stimulator device with neither cables nor inductive couplingto provide power.

In various implementations, the wireless implantable stimulator deviceis powered wirelessly (and therefore does not require a wiredconnection) and contains the circuitry necessary to receive the pulseinstructions from a source external to the body. For example, variousembodiments employ internal dipole (or other) antenna configuration(s)to receive RF power through electrical radiative coupling. This allowssuch devices to produce electrical currents capable of stimulating nervebundles without a physical connection to an implantable pulse generator(IPG) or use of an inductive coil.

According to some implementations, the wireless implantable stimulatordevice includes an application-specific integrated circuit (ASIC) chipfor interacting with an external controller and the electrodes containedwithin the device. The ASIC chip may harvest RF power from the receivedinput signal sent from the external controller to power the wirelessimplantable stimulator device, including the ASIC chip. The ASIC chipmay also extract waveform parameters from the received input signal anduse such information to create electrical impulses for stimulatingexcitable tissues through the electrodes. In particular, the ASIC chipcontains a current steering feature to mirror currents to each electrodewith evenness while maintaining a compact chip size. Moreover, the ASICchip may extract polarity setting information from the received inputsignal and use such information to set the polarity for electrodeinterfaces.

Further descriptions of exemplary wireless systems for providing neuralstimulation to a patient can be found in commonly-assigned, co-pendingpublished PCT applications PCT/US2012/23029 filed Jan. 28, 2011,PCT/US2012/32200 filed Apr. 11, 2011, PCT/US2012/48903, filed Jan. 28,2011, PCT/US2012/50633, filed Aug. 12, 2011 and PCT/US2012/55746, filedSep. 15, 2011, the complete disclosures of which are incorporated byreference.

FIG. 1 depicts a high-level diagram of an example of a wirelessstimulation system. The wireless stimulation system may include fourmajor components, namely, a programmer module 102, a RF pulse generatormodule 106, a transmit (TX) antenna 110 (for example, a patch antenna,slot antenna, or a dipole antenna), and an implanted wireless stimulatordevice 114. The programmer module 102 may be a computer device, such asa smart phone, running a software application that supports a wirelessconnection 104, such as Bluetooth®. The application can enable the userto view the system status and diagnostics, change various parameters,increase/decrease the desired stimulus amplitude of the electrodepulses, and adjust feedback sensitivity of the RF pulse generator module106, among other functions.

The RF pulse generator module 106 may include communication electronicsthat support the wireless connection 104, the stimulation circuitry, andthe battery to power the generator electronics. In some implementations,the RF pulse generator module 106 includes the TX antenna embedded intoits packaging form factor while, in other implementations, the TXantenna is connected to the RF pulse generator module 106 through awired connection 108 or a wireless connection (not shown). The TXantenna 110 may be coupled directly to tissue to create an electricfield that powers the implanted wireless stimulator device 114. The TXantenna 110 communicates with the implanted wireless stimulator device114 through an RF interface. For instance, the TX antenna 110 radiatesan RF transmission signal that is modulated and encoded by the RF pulsegenerator module 110. The implanted wireless stimulator device of module114 contains one or more antennas, such as dipole antenna(s), to receiveand transmit through RF interface 112. In particular, the couplingmechanism between antenna 110 and the one or more antennas on theimplanted wireless stimulation device of module 114 utilizes electricalradiative coupling and not inductive coupling. In other words, thecoupling is through an electric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna 110 canprovide an input signal to the implanted wireless stimulator device 114.This input signal contains energy and may contain information encodingstimulus waveforms to be applied at the electrodes of the implantedwireless stimulator device 114. In some implementations, the power levelof this input signal directly determines an applied amplitude (forexample, power, current, or voltage) of the one or more electricalpulses created using the electrical energy contained in the inputsignal. Within the implanted wireless stimulator device 114 arecomponents for demodulating the RF transmission signal, and electrodesto deliver the stimulation to surrounding neuronal tissue.

The RF pulse generator module 106 can be implanted subcutaneously, or itcan be worn external to the body. When external to the body, the RFgenerator module 106 can be incorporated into a belt or harness designto allow for electric radiative coupling through the skin and underlyingtissue to transfer power and/or control parameters to the implantedwireless stimulator device 114. In either event, receiver circuit(s)internal to the wireless stimulator device 114 (or cylindrical wirelessimplantable stimulator device 1400 shown in FIGS. 14A and 14B, helicalwireless implantable stimulator device 1900 shown in FIGS. 19A to 19C)can capture the energy radiated by the TX antenna 110 and convert thisenergy to an electrical waveform. The receiver circuit(s) may furthermodify the waveform to create an electrical pulse suitable for thestimulation of neural tissue.

In some implementations, the RF pulse generator module 106 can remotelycontrol the stimulus parameters (that is, the parameters of theelectrical pulses applied to the neural tissue) and monitor feedbackfrom the wireless stimulator device 114 based on RF signals receivedfrom the implanted wireless stimulator device 114. A feedback detectionalgorithm implemented by the RF pulse generator module 106 can monitordata sent wirelessly from the implanted wireless stimulator device 114,including information about the energy that the implanted wirelessstimulator device 114 is receiving from the RF pulse generator andinformation about the stimulus waveform being delivered to the electrodepads. In order to provide an effective therapy for a given medicalcondition, the system can be tuned to provide the optimal amount ofexcitation or inhibition to the nerve fibers by electrical stimulation.A closed loop feedback control method can be used in which the outputsignals from the implanted wireless stimulator device 114 are monitoredand used to determine the appropriate level of neural stimulationcurrent for maintaining effective neuronal activation, or, in somecases, the patient can manually adjust the output signals in an openloop control method.

FIG. 2 depicts a detailed diagram of an example of the wirelessstimulation system. As depicted, the programming module 102 may compriseuser input system 202 and communication subsystem 208. The user inputsystem 221 may allow various parameter settings to be adjusted (in somecases, in an open loop fashion) by the user in the form of instructionsets. The communication subsystem 208 may transmit these instructionsets (and other information) via the wireless connection 104, such asBluetooth or Wi-Fi, to the RF pulse generator module 106, as well asreceive data from module 106.

For instance, the programmer module 102, which can be utilized formultiple users, such as a patient's control unit or clinician'sprogrammer unit, can be used to send stimulation parameters to the RFpulse generator module 106. The stimulation parameters that can becontrolled may include pulse amplitude, pulse frequency, and pulse widthin the ranges shown in Table 1. In this context the term pulse refers tothe phase of the waveform that directly produces stimulation of thetissue; the parameters of the charge-balancing phase (described below)can similarly be controlled. The patient and/or the clinician can alsooptionally control overall duration and pattern of treatment.

Stimulation Parameter Table 1

Pulse Amplitude: 0 to 20 mA

Pulse Frequency: 0 to 10000 Hz

Pulse Width: 0 to 2 ms

The RF pulse generator module 106 may be initially programmed to meetthe specific parameter settings for each individual patient during theinitial implantation procedure. Because medical conditions or the bodyitself can change over time, the ability to re-adjust the parametersettings may be beneficial to ensure ongoing efficacy of the neuralmodulation therapy.

The programmer module 102 may be functionally a smart device andassociated application. The smart device hardware may include a CPU 206and be used as a vehicle to handle touchscreen input on a graphical userinterface (GUI) 204, for processing and storing data.

The RF pulse generator module 106 may be connected via wired connection108 to an external TX antenna 110. Alternatively, both the antenna andthe RF pulse generator are located subcutaneously (not shown).

The signals sent by RF pulse generator module 106 to the implantedwireless stimulator device 114 may include both power andparameter-setting attributes in regards to stimulus waveform, amplitude,pulse width, and frequency. The RF pulse generator module 106 can alsofunction as a wireless receiving unit that receives feedback signalsfrom the implanted wireless stimulator device 114. To that end, the RFpulse generator module 106 may contain microelectronics or othercircuitry to handle the generation of the signals transmitted to thedevice 114 as well as handle feedback signals, such as those from thestimulator device 114. For example, the RF pulse generator module 106may comprise controller subsystem 214, high-frequency oscillator 218, RFamplifier 216, a RF switch, and a feedback subsystem 212.

The controller subsystem 214 may include a CPU 230 to handle dataprocessing, a memory subsystem 228 such as a local memory, communicationsubsystem 234 to communicate with programmer module 102 (includingreceiving stimulation parameters from programmer module), pulsegenerator circuitry 236, and digital/analog (D/A) converters 232.

The controller subsystem 214 may be used by the patient and/or theclinician to control the stimulation parameter settings (for example, bycontrolling the parameters of the signal sent from RF pulse generatormodule 106 to the stimulator device 114). These parameter settings canaffect, for example, the power, current level, or shape of the one ormore electrical pulses. The programming of the stimulation parameterscan be performed using the programming module 102, as described above,to set the repetition rate, pulse width, amplitude, and waveform thatwill be transmitted by RF energy to the receiving (RX) antenna 238,typically a dipole antenna (although other types may be used), in theimplanted wireless stimulation device 214. The clinician may have theoption of locking and/or hiding certain settings within the programmerinterface, thus limiting the patient's ability to view or adjust certainparameters because adjustment of certain parameters may require detailedmedical knowledge of neurophysiology, neuroanatomy, protocols for neuralmodulation, and safety limits of electrical stimulation.

The controller subsystem 214 may store received parameter settings inthe local memory subsystem 228, until the parameter settings aremodified by new input data received from the programming module 102. TheCPU 206 may use the parameters stored in the local memory to control thepulse generator circuitry 236 to generate a stimulus waveform that ismodulated by a high frequency oscillator 218 in the range from 300 MHzto 8 GHz (preferably between about 700 MHz and 5.8 GHz and morepreferably between about 800 MHz and 1.3 GHz). The resulting RF signalmay then be amplified by RF amplifier 226 and then sent through an RFswitch 223 to the TX antenna 110 to reach through depths of tissue tothe RX antenna 238.

In some implementations, the RF signal sent by TX antenna 110 may simplybe a power transmission signal used by the wireless stimulation devicemodule 114 to generate electric pulses. In other implementations, atelemetry signal may also be transmitted to the wireless stimulatordevice 114 to send instructions about the various operations of thewireless stimulator device 114. The telemetry signal may be sent by themodulation of the carrier signal (through the skin if external, orthrough other body tissues if the pulse generator module 106 isimplanted subcutaneously). The telemetry signal is used to modulate thecarrier signal (a high frequency signal) that is coupled onto theimplanted antenna(s) 238 and does not interfere with the input receivedon the same stimulator device to power the device. In one embodiment thetelemetry signal and powering signal are combined into one signal, wherethe RF telemetry signal is used to modulate the RF powering signal, andthus the wireless stimulation device is powered directly by the receivedtelemetry signal; separate subsystems in the wireless stimulation deviceharness the power contained in the signal and interpret the data contentof the signal.

The RF switch 223 may be a multipurpose device such as a dualdirectional coupler, which passes the relatively high amplitude,extremely short duration RF pulse to the TX antenna 110 with minimalinsertion loss while simultaneously providing two low-level outputs tofeedback subsystem 212; one output delivers a forward power signal tothe feedback subsystem 212, where the forward power signal is anattenuated version of the RF pulse sent to the TX antenna 110, and theother output delivers a reverse power signal to a different port of thefeedback subsystem 212, where reverse power is an attenuated version ofthe reflected RF energy from the TX Antenna 110.

During the on-cycle time (when an RF signal is being transmitted towireless stimulator device 114), the RF switch 223 is set to send theforward power signal to feedback subsystem. During the off-cycle time(when an RF signal is not being transmitted to the wireless stimulatordevice 114), the RF switch 223 can change to a receiving mode in whichthe reflected RF energy and/or RF signals from the wireless stimulatordevice 114 are received to be analyzed in the feedback subsystem 212.

The feedback subsystem 212 of the RF pulse generator module 106 mayinclude reception circuitry to receive and extract telemetry or otherfeedback signals from the wireless stimulator device 114 and/orreflected RF energy from the signal sent by TX antenna 110. The feedbacksubsystem may include an amplifier 226, a filter 224, a demodulator 222,and an A/D converter 220.

The feedback subsystem 212 receives the forward power signal andconverts this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. In this way thecharacteristics of the generated RF pulse can be compared to a referencesignal within the controller subsystem 214. If a disparity (error)exists in any parameter, the controller subsystem 214 can adjust theoutput to the RF pulse generator 106. The nature of the adjustment canbe, for example, proportional to the computed error. The controllersubsystem 214 can incorporate additional inputs and limits on itsadjustment scheme such as the signal amplitude of the reverse power andany predetermined maximum or minimum values for various pulseparameters.

The reverse power signal can be used to detect fault conditions in theRF-power delivery system. In an ideal condition, when TX antenna 110 hasperfectly matched impedance to the tissue that it contacts, theelectromagnetic waves generated from the RF pulse generator 106 passunimpeded from the TX antenna 110 into the body tissue. However, inreal-world applications a large degree of variability may exist in thebody types of users, types of clothing worn, and positioning of theantenna 110 relative to the body surface. Since the impedance of theantenna 110 depends on the relative permittivity of the underlyingtissue and any intervening materials, and also depends on the overallseparation distance of the antenna from the skin, in any givenapplication there can be an impedance mismatch at the interface of theTX antenna 110 with the body surface. When such a mismatch occurs, theelectromagnetic waves sent from the RF pulse generator 106 are partiallyreflected at this interface, and this reflected energy propagatesbackward through the antenna feed.

The dual directional coupler RF switch 223 may prevent the reflected RFenergy propagating back into the amplifier 226, and may attenuate thisreflected RF signal and send the attenuated signal as the reverse powersignal to the feedback subsystem 212. The feedback subsystem 212 canconvert this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. The controller subsystem 214can then calculate the ratio of the amplitude of the reverse powersignal to the amplitude of the forward power signal. The ratio of theamplitude of reverse power signal to the amplitude level of forwardpower may indicate severity of the impedance mismatch.

In order to sense impedance mismatch conditions, the controllersubsystem 214 can measure the reflected-power ratio in real time, andaccording to preset thresholds for this measurement, the controllersubsystem 214 can modify the level of RF power generated by the RF pulsegenerator 106. For example, for a moderate degree of reflected power thecourse of action can be for the controller subsystem 214 to increase theamplitude of RF power sent to the TX antenna 110, as would be needed tocompensate for slightly non-optimum but acceptable TX antenna couplingto the body. For higher ratios of reflected power, the course of actioncan be to prevent operation of the RF pulse generator 106 and set afault code to indicate that the TX antenna 110 has little or no couplingwith the body. This type of reflected-power fault condition can also begenerated by a poor or broken connection to the TX antenna. In eithercase, it may be desirable to stop RF transmission when thereflected-power ratio is above a defined threshold, because internallyreflected power can lead to unwanted heating of internal components, andthis fault condition means the system cannot deliver sufficient power tothe implanted wireless stimulation device and thus cannot delivertherapy to the user.

The controller 242 of the wireless stimulator device 114 may transmitinformational signals, such as a telemetry signal, through the antenna238 to communicate with the RF pulse generator module 106 during itsreceive cycle. For example, the telemetry signal from the wirelessstimulator device 114 may be coupled to the modulated signal on thedipole antenna(s) 238, during the on and off state of the transistorcircuit to enable or disable a waveform that produces the correspondingRF bursts necessary to transmit to the external (or remotely implanted)pulse generator module 106. The antenna(s) 238 may be connected toelectrodes 254 in contact with tissue to provide a return path for thetransmitted signal. An A/D (not shown) converter can be used to transferstored data to a serialized pattern that can be transmitted on thepulse-modulated signal from the internal antenna(s) 238 of the wirelessstimulator device 114.

A telemetry signal from the implanted wireless stimulator device 114 mayinclude stimulus parameters such as the power or the amplitude of thecurrent that is delivered to the tissue from the electrodes. Thefeedback signal can be transmitted to the RF pulse generator module 116to indicate the strength of the stimulus at the nerve bundle by means ofcoupling the signal to the implanted RX antenna 238, which radiates thetelemetry signal to the external (or remotely implanted) RF pulsegenerator module 106. The feedback signal can include either or both ananalog and digital telemetry pulse modulated carrier signal. Data suchas stimulation pulse parameters and measured characteristics ofstimulator performance can be stored in an internal memory device withinthe implanted stimulator device 114, and sent on the telemetry signal.The frequency of the carrier signal may be in the range of at 300 MHz to8 GHz (preferably between about 700 MHz and 5.8 GHz and more preferablybetween about 800 MHz and 1.3 GHz).

In the feedback subsystem 212, the telemetry signal can be downmodulated using demodulator 222 and digitized by being processed throughan analog to digital (A/D) converter 220. The digital telemetry signalmay then be routed to a CPU 230 with embedded code, with the option toreprogram, to translate the signal into a corresponding currentmeasurement in the tissue based on the amplitude of the received signal.The CPU 230 of the controller subsystem 214 can compare the reportedstimulus parameters to those held in local memory 228 to verify thewireless stimulator device 114 delivered the specified stimuli totissue. For example, if the wireless stimulation device reports a lowercurrent than was specified, the power level from the RF pulse generatormodule 106 can be increased so that the implanted wireless stimulatordevice 114 will have more available power for stimulation. The implantedwireless stimulator device 114 can generate telemetry data in real time,for example, at a rate of 8 Kbits per second. All feedback data receivedfrom the implanted stimulator device 114 can be logged against time andsampled to be stored for retrieval to a remote monitoring systemaccessible by the health care professional for trending and statisticalcorrelations.

The sequence of remotely programmable RF signals received by theinternal antenna(s) 238 may be conditioned into waveforms that arecontrolled within the implantable wireless stimulator device 114 by thecontrol subsystem 242 and routed to the appropriate electrodes 254 thatare placed in proximity to the tissue to be stimulated. For instance,the RF signal transmitted from the RF pulse generator module 106 may bereceived by RX antenna 238 and processed by circuitry, such as waveformconditioning circuitry 240, within the implanted wireless stimulatordevice 114 to be converted into electrical pulses applied to theelectrodes 254 through electrode interface 252. In some implementations,the implanted wireless stimulator device 114 contains between two tosixteen electrodes 254.

The waveform conditioning circuitry 240 may include a rectifier 244,which rectifies the signal received by the RX antenna 238. The rectifiedsignal may be fed to the controller 242 for receiving encodedinstructions from the RF pulse generator module 106. The rectifiersignal may also be fed to a charge balance component 246 that isconfigured to create one or more electrical pulses based such that theone or more electrical pulses result in a substantially zero net chargeat the one or more electrodes (that is, the pulses are charge balanced).The charge-balanced pulses are passed through the current limiter 248 tothe electrode interface 252, which applies the pulses to the electrodes254 as appropriate.

The current limiter 248 insures the current level of the pulses appliedto the electrodes 254 is not above a threshold current level. In someimplementations, an amplitude (for example, current level, voltagelevel, or power level) of the received RF pulse directly determines theamplitude of the stimulus. In this case, it may be particularlybeneficial to include current limiter 248 to prevent excessive currentor charge being delivered through the electrodes, although currentlimiter 248 may be used in other implementations where this is not thecase. Generally, for a given electrode having several square millimeterssurface area, it is the charge per phase that should be limited forsafety (where the charge delivered by a stimulus phase is the integralof the current). But, in some cases, the limit can instead be placed onthe current, where the maximum current multiplied by the maximumpossible pulse duration is less than or equal to the maximum safecharge. More generally, the limiter 248 acts as a charge limiter thatlimits a characteristic (for example, current or duration) of theelectrical pulses so that the charge per phase remains below a thresholdlevel (typically, a safe-charge limit).

In the event the implanted wireless stimulator device 114 receives a“strong” pulse of RF power sufficient to generate a stimulus that wouldexceed the predetermined safe-charge limit, the current limiter 248 canautomatically limit or “clip” the stimulus phase to maintain the totalcharge of the phase within the safety limit. The current limiter 248 maybe a passive current limiting component that cuts the signal to theelectrodes 254 once the safe current limit (the threshold current level)is reached. Alternatively, or additionally, the current limiter 248 maycommunicate with the electrode interface 252 to turn off all electrodes254 to prevent tissue damaging current levels.

A clipping event may trigger a current limiter feedback control mode.The action of clipping may cause the controller to send a thresholdpower data signal to the pulse generator 106. The feedback subsystem 212detects the threshold power signal and demodulates the signal into datathat is communicated to the controller subsystem 214. The controllersubsystem 214 algorithms may act on this current-limiting condition byspecifically reducing the RF power generated by the RF pulse generator,or cutting the power completely. In this way, the pulse generator 106can reduce the RF power delivered to the body if the implanted wirelessstimulator device 114 reports it is receiving excess RF power.

The controller 250 of the stimulator 205 may communicate with theelectrode interface 252 to control various aspects of the electrodesetup and pulses applied to the electrodes 254. The electrode interface252 may act as a multiplex and control the polarity and switching ofeach of the electrodes 254. For instance, in some implementations, thewireless stimulator 106 has multiple electrodes 254 in contact withtissue, and for a given stimulus the RF pulse generator module 106 canarbitrarily assign one or more electrodes to 1) act as a stimulatingelectrode, 2) act as a return electrode, or 3) be inactive bycommunication of assignment sent wirelessly with the parameterinstructions, which the controller 250 uses to set electrode interface252 as appropriate. It may be physiologically advantageous to assign,for example, one or two electrodes as stimulating electrodes and toassign all remaining electrodes as return electrodes.

Also, in some implementations, for a given stimulus pulse, thecontroller 250 may control the electrode interface 252 to divide thecurrent arbitrarily (or according to instructions from pulse generatormodule 106) among the designated stimulating electrodes. This controlover electrode assignment and current control can be advantageousbecause in practice the electrodes 254 may be spatially distributedalong various neural structures, and through strategic selection of thestimulating electrode location and the proportion of current specifiedfor each location, the aggregate current distribution in tissue can bemodified to selectively activate specific neural targets. This strategyof current steering can improve the therapeutic effect for the patient.

In another implementation, the time course of stimuli may be arbitrarilymanipulated. A given stimulus waveform may be initiated at a time Tstart and terminated at a time T final, and this time course may besynchronized across all stimulating and return electrodes; further, thefrequency of repetition of this stimulus cycle may be synchronous forall the electrodes. However, controller 250, on its own or in responseto instructions from pulse generator 106, can control electrodeinterface 252 to designate one or more subsets of electrodes to deliverstimulus waveforms with non-synchronous start and stop times, and thefrequency of repetition of each stimulus cycle can be arbitrarily andindependently specified.

For example, a stimulator having eight electrodes may be configured tohave a subset of five electrodes, called set A, and a subset of threeelectrodes, called set B. Set A might be configured to use two of itselectrodes as stimulating electrodes, with the remainder being returnelectrodes. Set B might be configured to have just one stimulatingelectrode. The controller 250 could then specify that set A deliver astimulus phase with 3 mA current for a duration of 200 us followed by a400 us charge-balancing phase. This stimulus cycle could be specified torepeat at a rate of 60 cycles per second. Then, for set B, thecontroller 250 could specify a stimulus phase with 1 mA current forduration of 500 us followed by a 800 us charge-balancing phase. Therepetition rate for the set-B stimulus cycle can be set independently ofset A, say for example it could be specified at 25 cycles per second.Or, if the controller 250 was configured to match the repetition ratefor set B to that of set A, for such a case the controller 250 canspecify the relative start times of the stimulus cycles to be coincidentin time or to be arbitrarily offset from one another by some delayinterval.

In some implementations, the controller 250 can arbitrarily shape thestimulus waveform amplitude, and may do so in response to instructionsfrom pulse generator 106. The stimulus phase may be delivered by aconstant-current source or a constant-voltage source, and this type ofcontrol may generate characteristic waveforms that are static, e.g. aconstant-current source generates a characteristic rectangular pulse inwhich the current waveform has a very steep rise, a constant amplitudefor the duration of the stimulus, and then a very steep return tobaseline. Alternatively, or additionally, the controller 250 canincrease or decrease the level of current at any time during thestimulus phase and/or during the charge-balancing phase. Thus, in someimplementations, the controller 250 can deliver arbitrarily shapedstimulus waveforms such as a triangular pulse, sinusoidal pulse, orGaussian pulse for example. Similarly, the charge-balancing phase can bearbitrarily amplitude-shaped, and similarly a leading anodic pulse(prior to the stimulus phase) may also be amplitude-shaped.

As described above, the wireless stimulator device 114 may include acharge-balancing component 246. Generally, for constant currentstimulation pulses, pulses should be charge balanced by having theamount of cathodic current should equal the amount of anodic current,which is typically called biphasic stimulation. Charge density is theamount of current times the duration it is applied, and is typicallyexpressed in the units uC/cm². In order to avoid the irreversibleelectrochemical reactions such as pH change, electrode dissolution aswell as tissue destruction, no net charge should appear at theelectrode-electrolyte interface, and it is generally acceptable to havea charge density less than 30 uC/cm². Biphasic stimulating currentpulses ensure that no net charge appears at the electrode after eachstimulation cycle and the electrochemical processes are balanced toprevent net dc currents. The wireless stimulator device 114 may bedesigned to ensure that the resulting stimulus waveform has a net zerocharge. Charge balanced stimuli are thought to have minimal damagingeffects on tissue by reducing or eliminating electrochemical reactionproducts created at the electrode-tissue interface.

A stimulus pulse may have a negative-voltage or current, called thecathodic phase of the waveform. Stimulating electrodes may have bothcathodic and anodic phases at different times during the stimulus cycle.An electrode that delivers a negative current with sufficient amplitudeto stimulate adjacent neural tissue is called a “stimulating electrode.”During the stimulus phase the stimulating electrode acts as a currentsink. One or more additional electrodes act as a current source andthese electrodes are called “return electrodes.” Return electrodes areplaced elsewhere in the tissue at some distance from the stimulatingelectrodes. When a typical negative stimulus phase is delivered totissue at the stimulating electrode, the return electrode has a positivestimulus phase. During the subsequent charge-balancing phase, thepolarities of each electrode are reversed.

In some implementations, the charge balance component 246 uses ablocking capacitor(s) placed electrically in series with the stimulatingelectrodes and body tissue, between the point of stimulus generationwithin the stimulator circuitry and the point of stimulus delivery totissue. In this manner, a resistor-capacitor (RC) network may be formed.In a multi-electrode stimulator, one charge-balance capacitor(s) may beused for each electrode or a centralized capacitor(s) may be used withinthe stimulator circuitry prior to the point of electrode selection. TheRC network can block direct current (DC), however it can also preventlow-frequency alternating current (AC) from passing to the tissue. Thefrequency below which the series RC network essentially blocks signalsis commonly referred to as the cutoff frequency, and in one embodimentthe design of the stimulator system may ensure the cutoff frequency isnot above the fundamental frequency of the stimulus waveform. In thisembodiment as disclosed herein, the wireless stimulator may have acharge-balance capacitor with a value chosen according to the measuredseries resistance of the electrodes and the tissue environment in whichthe stimulator is implanted. By selecting a specific capacitance valuethe cutoff frequency of the RC network in this embodiment is at or belowthe fundamental frequency of the stimulus pulse.

In other implementations, the cutoff frequency may be chosen to be at orabove the fundamental frequency of the stimulus, and in this scenariothe stimulus waveform created prior to the charge-balance capacitor,called the drive waveform, may be designed to be non-stationary, wherethe envelope of the drive waveform is varied during the duration of thedrive pulse. For example, in one embodiment, the initial amplitude ofthe drive waveform is set at an initial amplitude Vi, and the amplitudeis increased during the duration of the pulse until it reaches a finalvalue k*Vi. By changing the amplitude of the drive waveform over time,the shape of the stimulus waveform passed through the charge-balancecapacitor is also modified. The shape of the stimulus waveform may bemodified in this fashion to create a physiologically advantageousstimulus.

In some implementations, the wireless stimulator device 114 may create adrive-waveform envelope that follows the envelope of the RF pulsereceived by the receiving dipole antenna(s) 238. In this case, the RFpulse generator module 106 can directly control the envelope of thedrive waveform within the wireless stimulator device 114, and thus noenergy storage may be required inside the stimulator itself. In thisimplementation, the stimulator circuitry may modify the envelope of thedrive waveform or may pass it directly to the charge-balance capacitorand/or electrode-selection stage.

In some implementations, the implanted wireless stimulator device 114may deliver a single-phase drive waveform to the charge balancecapacitor or it may deliver multiphase drive waveforms. In the case of asingle-phase drive waveform, for example, a negative-going rectangularpulse, this pulse comprises the physiological stimulus phase, and thecharge-balance capacitor is polarized (charged) during this phase. Afterthe drive pulse is completed, the charge balancing function is performedsolely by the passive discharge of the charge-balance capacitor, whereis dissipates its charge through the tissue in an opposite polarityrelative to the preceding stimulus. In one implementation, a resistorwithin the stimulator facilitates the discharge of the charge-balancecapacitor. In some implementations, using a passive discharge phase, thecapacitor may allow virtually complete discharge prior to the onset ofthe subsequent stimulus pulse.

In the case of multiphase drive waveforms the wireless stimulator mayperform internal switching to pass negative-going or positive-goingpulses (phases) to the charge-balance capacitor. These pulses may bedelivered in any sequence and with varying amplitudes and waveformshapes to achieve a desired physiological effect. For example, thestimulus phase may be followed by an actively driven charge-balancingphase, and/or the stimulus phase may be preceded by an opposite phase.Preceding the stimulus with an opposite-polarity phase, for example, canhave the advantage of reducing the amplitude of the stimulus phaserequired to excite tissue.

In some implementations, the amplitude and timing of stimulus andcharge-balancing phases is controlled by the amplitude and timing of RFpulses from the RF pulse generator module 106, and in others thiscontrol may be administered internally by circuitry onboard the wirelessstimulator device 114, such as controller 250. In the case of onboardcontrol, the amplitude and timing may be specified or modified by datacommands delivered from the pulse generator module 106.

FIG. 3 is a flowchart showing an example of an operation of a wirelessneural stimulation system. In block 302, the wireless stimulator device114 is implanted in proximity to nerve bundles and is coupled to theelectric field produced by the TX antenna 110. That is, the pulsegenerator module 106 and the TX antenna 110 are positioned in such a way(for example, in proximity to the patient) that the TX antenna 110 iselectrically radiatively coupled with the implanted RX antenna 238 ofthe wireless stimulator device 114. In certain implementations, both theantenna 110 and the RF pulse generator 106 are located subcutaneously.In other implementations, the antenna 110 and the RF pulse generator 106are located external to the patient's body. In this case, the TX antenna110 may be coupled directly to the patient's skin.

Energy from the RF pulse generator is radiated to the implanted wirelessstimulator device 114 from the antenna 110 through tissue, as shown inblock 304. The energy radiated may be controlled by thePatient/Clinician Parameter inputs in block 301. In some instances, theparameter settings can be adjusted in an open loop fashion by thepatient or clinician, who would adjust the parameter inputs in block 301to the system.

The implanted wireless stimulator device 114 uses the received energy togenerate electrical pulses to be applied to the neural tissue throughthe electrodes 238. For instance, the wireless stimulator device 114 maycontain circuitry that rectifies the received RF energy and conditionsthe waveform to charge balance the energy delivered to the electrodes tostimulate the targeted nerves or tissues, as shown in block 306. Theimplanted wireless stimulator device 114 communicates with the pulsegenerator 106 by using antenna 238 to send a telemetry signal, as shownin block 308. The telemetry signal may contain information aboutparameters of the electrical pulses applied to the electrodes, such asthe impedance of the electrodes, whether the safe current limit has beenreached, or the amplitude of the current that is presented to the tissuefrom the electrodes.

In block 310, the RF pulse generator 106 detects amplifies, filters andmodulates the received telemetry signal using amplifier 226, filter 224,and demodulator 222, respectively. The A/D converter 230 then digitizesthe resulting analog signal, as shown in 312. The digital telemetrysignal is routed to CPU 230, which determines whether the parameters ofthe signal sent to the wireless stimulator device 114 need to beadjusted based on the digital telemetry signal. For instance, in block314, the CPU 230 compares the information of the digital signal to alook-up table, which may indicate an appropriate change in stimulationparameters. The indicated change may be, for example, a change in thecurrent level of the pulses applied to the electrodes. As a result, theCPU may change the output power of the signal sent to wirelessstimulator device 114 so as to adjust the current applied by theelectrodes 254, as shown in block 316.

Thus, for instance, the CPU 230 may adjust parameters of the signal sentto the wireless stimulator device 114 every cycle to match the desiredcurrent amplitude setting programmed by the patient, as shown in block318. The status of the stimulator system may be sampled in real time ata rate of 8 Kbits per second of telemetry data. All feedback datareceived from the wireless stimulator device 114 can be maintainedagainst time and sampled per minute to be stored for download or uploadto a remote monitoring system accessible by the health care professionalfor trending and statistical correlations in block 318. If operated inan open loop fashion, the stimulator system operation may be reduced tojust the functional elements shown in blocks 302, 304, 306, and 308, andthe patient uses their judgment to adjust parameter settings rather thanthe closed looped feedback from the implanted device.

FIG. 4 is a circuit diagram showing an example of a wireless stimulatordevice 114. This example contains paired electrodes, comprising cathodeelectrode(s) 408 and anode electrode(s) 410, as shown. When energized,the charged electrodes create a volume conduction field of currentdensity within the tissue. In this implementation, the wireless energyis received through a dipole antenna(s) 238. At least four diodes areconnected together to form a full wave bridge rectifier 402 attached tothe dipole antenna(s) 238. Each diode, up to 100 micrometers in length,uses a junction potential to prevent the flow of negative electricalcurrent, from cathode to anode, from passing through the device whensaid current does not exceed the reverse threshold. For neuralstimulation via wireless power, transmitted through tissue, the naturalinefficiency of the lossy material may lead to a low threshold voltage.In this implementation, a zero biased diode rectifier results in a lowoutput impedance for the device. A resistor 404 and a smoothingcapacitor 406 are placed across the output nodes of the bridge rectifierto discharge the electrodes to the ground of the bridge anode. Therectification bridge 402 includes two branches of diode pairs connectingan anode-to-anode and then cathode to cathode. The electrodes 408 and410 are connected to the output of the charge balancing circuit 246.

FIG. 5 is a circuit diagram of another example of a wireless stimulatordevice 114. The example shown in FIG. 5 includes multiple electrodecontrol and may employ full closed loop control. The wirelessstimulation device includes an electrode array 254 in which the polarityof the electrodes can be assigned as cathodic or anodic, and for whichthe electrodes can be alternatively not powered with any energy. Whenenergized, the charged electrodes create a volume conduction field ofcurrent density within the tissue. In this implementation, the wirelessenergy is received by the device through the dipole antenna(s) 238. Theelectrode array 254 is controlled through an on-board controller circuit242 that sends the appropriate bit information to the electrodeinterface 252 in order to set the polarity of each electrode in thearray, as well as power to each individual electrode. The lack of powerto a specific electrode would set that electrode in a functional OFFposition. In another implementation (not shown), the amount of currentsent to each electrode is also controlled through the controller 242.The controller current, polarity and power state parameter data, shownas the controller output, is be sent back to the antenna(s) 238 fortelemetry transmission back to the pulse generator module 106. Thecontroller 242 also includes the functionality of current monitoring andsets a bit register counter so that the status of total current drawncan be sent back to the pulse generator module 106.

At least four diodes can be connected together to form a full wavebridge rectifier 302 attached to the dipole antenna(s) 238. Each diode,up to 100 micrometers in length, uses a junction potential to preventthe flow of negative electrical current, from cathode to anode, frompassing through the device when said current does not exceed the reversethreshold. For neural stimulation via wireless power, transmittedthrough tissue, the natural inefficiency of the lossy material may leadto a low threshold voltage. In this implementation, a zero biased dioderectifier results in a low output impedance for the device. A resistor404 and a smoothing capacitor 406 are placed across the output nodes ofthe bridge rectifier to discharge the electrodes to the ground of thebridge anode. The rectification bridge 402 may include two branches ofdiode pairs connecting an anode-to-anode and then cathode to cathode.The electrode polarity outputs, both cathode 408 and anode 410 areconnected to the outputs formed by the bridge connection. Chargebalancing circuitry 246 and current limiting circuitry 248 are placed inseries with the outputs.

FIG. 6 is a block diagram showing an example of control functions 605and feedback functions 630 of an implantable wireless stimulator device600, such as the ones described above or further below. An exampleimplementation may be a wireless stimulator device module 114, asdiscussed above in association with FIG. 2. Control functions 605include functions 610 for polarity switching of the electrodes andfunctions 620 for power-on reset.

Polarity switching functions 610 may employ, for example, a polarityrouting switch network to assign polarities to electrodes 254. Theassignment of polarity to an electrode may, for instance, be one of: acathode (negative polarity), an anode (positive polarity), or a neutral(off) polarity. The polarity assignment information for each of theelectrodes 254 may be contained in the input signal received byimplantable wireless stimulator device 600 through Rx antenna 238 fromRF pulse generator module 106. Because a programmer module 102 maycontrol RF pulse generator module 106, the polarity of electrodes 254may be controlled remotely by a programmer through programmer module102, as shown in FIG. 2.

Power-on reset functions 620 may reset the polarity assignment of eachelectrode immediately on each power-on event. As will be described infurther detail below, this reset operation may cause RF pulse generatormodule 106 to transmit the polarity assignment information to theimplantable wireless stimulator device 600. Once the polarity assignmentinformation is received by the implantable wireless stimulator device600, the polarity assignment information may be stored in a registerfile, or other short-term memory component. Thereafter the polarityassignment information may be used to configure the polarity assignmentof each electrode. If the polarity assignment information transmitted inresponse to the reset encodes the same polarity state as before thepower-on event, then the polarity state of each electrode can bemaintained before and after each power-on event.

Feedback functions 630 include functions 640 for monitoring deliveredpower to electrodes 254 and functions 650 for making impedance diagnosisof electrodes 254. For example, delivered power functions 640 mayprovide data encoding the amount of power being delivered fromelectrodes 254 to the excitable tissue and tissue impedance diagnosticfunctions 650 may provide data encoding the diagnostic information oftissue impedance. The tissue impedance is the electrical impedance ofthe tissue as seen between negative and positive electrodes when astimulation current is being released between negative and positiveelectrodes.

Feedback functions 630 may additionally include tissue depth estimatefunctions 660 to provide data indicating the overall tissue depth thatthe input radio frequency (RF) signal from the pulse generator module,such as, for example, RF pulse generator module 106, has penetratedbefore reaching the implanted antenna, such as, for example, RX antenna238, within the wireless implantable stimulator device 600, such as, forexample, implanted wireless stimulator device 114. For instance, thetissue depth estimate may be provided by comparing the power of thereceived input signal to the power of the RF pulse transmitted by the RFpulse generator 106. The ratio of the power of the received input signalto the power of the RF pulse transmitted by the RF pulse generator 106may indicate an attenuation caused by wave propagation through thetissue. For example, the second harmonic described below may be receivedby the RF pulse generator 106 and used with the power of the inputsignal sent by the RF pulse generator to determine the tissue depth. Theattenuation may be used to infer the overall depth of implantablewireless stimulator device 600 underneath the skin.

The data from blocks 640, 650, and 660 may be transmitted, for example,through Tx antenna 110 to an implantable RF pulse generator 106, asillustrated in FIGS. 1 and 2.

As discussed above in association with FIGS. 1, 2, 4, and 5, animplantable wireless stimulator device 600 may utilize rectificationcircuitry to convert the input signal (e.g., having a carrier frequencywithin a range from about 300 MHz to about 8 GHz) to a direct current(DC) power to drive the electrodes 254. Some implementations may providethe capability to regulate the DC power remotely. Some implementationsmay further provide different amounts of power to different electrodes,as discussed in further detail below.

FIG. 7 is a schematic showing an example of an implantable wirelessstimulator device 700 with components to implement control and feedbackfunctions as discussed above in association with FIG. 6. An RX antenna705 receives the input signal. The RX antenna 705 may be embedded as adipole, microstrip, folded dipole or other antenna configuration otherthan a coiled configuration, as described above. The input signal has acarrier frequency in the GHz range and contains electrical energy forpowering the wireless implantable stimulator device 700 and forproviding stimulation pulses to electrodes 254. Once received by theantenna 705, the input signal is routed to power management circuitry710. Power management circuitry 710 is configured to rectify the inputsignal and convert it to a DC power source. For example, the powermanagement circuitry 710 may include a diode rectification bridge suchas the diode rectification bridge 402 illustrated in FIG. 4. The DCpower source provides power to stimulation circuitry 711 and logic powercircuitry 713. The rectification may utilize one or more full wave diodebridge rectifiers within the power management circuitry 710. In oneimplementation, a resistor can be placed across the output nodes of thebridge rectifier to discharge the electrodes to the ground of the bridgeanode, as illustrated by the shunt register 404 in FIG. 7.

Turning momentarily to FIG. 8, a schematic of an example of a polarityrouting switch network 800 is shown. As discussed above, the cathodic(−) energy and the anodic energy are received at input 1 (block 722) andinput 2 (block 723), respectively. Polarity routing switch network 800has one of its outputs coupled to an electrode of electrodes 254 whichcan include as few as two electrodes, or as many as sixteen electrodes.Eight electrodes are shown in this implementation as an example.

Polarity routing switch network 800 is configured to either individuallyconnect each output to one of input 1 or input 2, or disconnect theoutput from either of the inputs. This selects the polarity for eachindividual electrode of electrodes 254 as one of: neutral (off), cathode(negative), or anode (positive). Each output is coupled to acorresponding three-state switch 830 for setting the connection state ofthe output. Each three-state switch is controlled by one or more of thebits from the selection input 850. In some implementations, selectioninput 850 may allocate more than one bits to each three-state switch.For example, two bits may encode the three-state information. Thus, thestate of each output of polarity routing switch device 800 can becontrolled by information encoding the bits stored in the register 732,which may be set by polarity assignment information received from theremote RF pulse generator module 106, as described further below.

Returning to FIG. 7, power and impedance sensing circuitry may be usedto determine the power delivered to the tissue and the impedance of thetissue. For example, a sensing resistor 718 may be placed in serialconnection with the anodic branch 714. Current sensing circuit 719senses the current across the resistor 718 and voltage sensing circuit720 senses the voltage across the resistor. The measured current andvoltage may correspond to the actual current and voltage applied by theelectrodes to the tissue.

As described below, the measured current and voltage may be provided asfeedback information to RF pulse generator module 106. The powerdelivered to the tissue may be determined by integrating the product ofthe measured current and voltage over the duration of the waveform beingdelivered to electrodes 254. Similarly, the impedance of the tissue maybe determined based on the measured voltage being applied to theelectrodes and the current being applied to the tissue. Alternativecircuitry (not shown) may also be used in lieu of the sensing resistor718, depending on implementation of the feature and whether bothimpedance and power feedback are measured individually, or combined.

The measurements from the current sensing circuitry 719 and the voltagesensing circuitry 720 may be routed to a voltage controlled oscillator(VCO) 733 or equivalent circuitry capable of converting from an analogsignal source to a carrier signal for modulation. VCO 733 can generate adigital signal with a carrier frequency. The carrier frequency may varybased on analog measurements such as, for example, a voltage, adifferential of a voltage and a power, etc. VCO 733 may also useamplitude modulation or phase shift keying to modulate the feedbackinformation at the carrier frequency. The VCO or the equivalent circuitmay be generally referred to as an analog controlled carrier modulator.The modulator may transmit information encoding the sensed current orvoltage back to RF pulse generator 106.

Antenna 725 may transmit the modulated signal, for example, in the GHzfrequency range, back to the RF pulse generator module 106. In someembodiments, antennas 705 and 725 may be the same physical antenna. Inother embodiments, antennas 705 and 725 may be separate physicalantennas. In the embodiments of separate antennas, antenna 725 mayoperate at a resonance frequency that is higher than the resonancefrequency of antenna 705 to send stimulation feedback to RF pulsegenerator module 106. In some embodiments, antenna 725 may also operateat the higher resonance frequency to receive data encoding the polarityassignment information from RF pulse generator module 106.

Antenna 725 may include a telemetry antenna 725 which may route receiveddata, such as polarity assignment information, to the stimulationfeedback circuit 730. The encoded polarity assignment information may beon a band in the GHz range. The received data may be demodulated bydemodulation circuitry 731 and then stored in the register file 732. Theregister file 732 may be a volatile memory. Register file 732 may be an8-channel memory bank that can store, for example, several bits of datafor each channel to be assigned a polarity. Some embodiments may have noregister file, while some embodiments may have a register file up to 64bits in size. The information encoded by these bits may be sent as thepolarity selection signal to polarity routing switch network 721, asindicated by arrow 734. The bits may encode the polarity assignment foreach output of the polarity routing switch network as one of: +(positive), − (negative), or 0 (neutral). Each output connects to oneelectrode and the channel setting determines whether the electrode willbe set as an anode (positive), cathode (negative), or off (neutral).

Returning to power management circuitry 710, in some embodiments,approximately 90% of the energy received is routed to the stimulationcircuitry 711 and less than 10% of the energy received is routed to thelogic power circuitry 713. Logic power circuitry 713 may power thecontrol components for polarity and telemetry. In some implementations,the power circuitry 713, however, does not provide the actual power tothe electrodes for stimulating the tissues. In certain embodiments, theenergy leaving the logic power circuitry 713 is sent to a capacitorcircuit 716 to store a certain amount of readily available energy. Thevoltage of the stored charge in the capacitor circuit 716 may be denotedas Vdc. Subsequently, this stored energy is used to power a power-onreset circuit 716 configured to send a reset signal on a power-on event.If the wireless implantable neural stimulator 700 loses power for acertain period of time, for example, in the range from about 1millisecond to over 10 milliseconds, the contents in the register file732 and polarity setting on polarity routing switch network 721 may bezeroed. The implantable wireless stimulation device 700 may lose power,for example, when it becomes less aligned with RF pulse generator module106. Using this stored energy, power-on reset circuit 740 may provide areset signal as indicated by arrow 717. This reset signal may causestimulation feedback circuit 730 to notify RF pulse generator module 106of the loss of power. For example, stimulation feedback circuit 730 maytransmit a telemetry feedback signal to RF pulse generator module 106 asa status notification of the power outage. This telemetry feedbacksignal may be transmitted in response to the reset signal andimmediately after power is back on wireless stimulation device 700. RFpulse generator module 106 may then transmit one or more telemetrypackets to implantable wireless stimulation device. The telemetrypackets contain polarity assignment information, which may be saved toregister file 732 and may be sent to polarity routing switch network721. Thus, polarity assignment information in register file 732 may berecovered from telemetry packets transmitted by RF pulse generatormodule 106 and the polarity assignment for each output of polarityrouting switch network 721 may be updated accordingly based on thepolarity assignment information.

The telemetry antenna 725 may transmit the telemetry feedback signalback to RF pulse generator module 106 at a frequency higher than thecharacteristic frequency of an RX antenna 705. In one implementation,the telemetry antenna 725 can have a heightened resonance frequency thatis the second harmonic of the characteristic frequency of RX antenna705. For example, the second harmonic may be utilized to transmit powerfeedback information regarding an estimate of the amount of power beingreceived by the electrodes. The feedback information may then be used bythe RF pulse generator in determining any adjustment of the power levelto be transmitted by the RF pulse generator 106. In a similar manner,the second harmonic energy can be used to detect the tissue depth. Thesecond harmonic transmission can be detected by an external antenna, forexample, on RF pulse generator module 106 that is tuned to the secondharmonic. As a general matter, power management circuitry 710 maycontain rectifying circuits that are non-linear device capable ofgenerating harmonic energies from input signal. Harvesting such harmonicenergy for transmitting telemetry feedback signal could improve theefficiency of implantable wireless stimulator device 700.

FIG. 9A is a diagram of an example implementation of a microwave fieldstimulator (MFS) 902 as part of a stimulation system utilizing animplantable wireless stimulator device 922. In this example, the MFS 902is external to a patient's body and may be placed within in closeproximity, for example, within 3 feet, to an implantable wirelessstimulator device 922. The RF pulse generator module 106 may be oneexample implementation of MFS 902. MFS 902 may be generally known as acontroller module. The implantable wireless stimulator device 922 is apassive device. The implantable wireless stimulator device 922 does nothave its own independent power source, rather it receives power for itsoperation from transmission signals emitted from a TX antenna powered bythe MFS 902, as discussed above.

In certain embodiments, the MFS 902 may communicate with a programmer912. The programmer 912 may be a mobile computing device, such as, forexample, a laptop, a smart phone, a tablet, etc. The communication maybe wired, using for example, a USB or firewire cable. The communicationmay also be wireless, utilizing for example, a bluetooth protocolimplemented by a transmitting blue tooth module 904, which communicateswith the host bluetooth module 914 within the programmer 912.

The MFS 902 may additionally communicate with wireless stimulator device922 by transmitting a transmission signal through a Tx antenna 907coupled to an amplifier 906. The transmission signal may propagatethrough skin and underlying tissues to arrive at the Rx antenna 923 ofthe wireless stimulator device 922. In some implementations, thewireless stimulator device 922 may transmit a telemetry feedback signalback to microwave field stimulator 902.

The microwave field stimulator 902 may include a microcontroller 908configured to manage the communication with a programmer 912 andgenerate an output signal. The output signal may be used by themodulator 909 to modulate a RF carrier signal. The frequency of thecarrier signal may be in the microwave range, for example, from about300 MHz to about 8 GHz, preferably from about 800 MHz to 1.3 GHz. Themodulated RF carrier signal may be amplified by an amplifier 906 toprovide the transmission signal for transmission to the wirelessstimulator device 922 through a TX antenna 907.

FIG. 9B is a diagram of another example of an implementation of amicrowave field stimulator 902 as part of a stimulation system utilizinga wireless stimulator device 922. In this example, the microwave fieldstimulator 902 may be embedded in the body of the patient, for example,subcutaneously. The embedded microwave field stimulator 902 may receivepower from a detached, remote wireless battery charger 932.

The power from the wireless battery charger 932 to the embeddedmicrowave field stimulator 902 may be transmitted at a frequency in theMHz or GHz range. The microwave field stimulator 902 shall be embeddedsubcutaneously at a very shallow depth (e.g., less than 1 cm), andalternative coupling methods may be used to transfer energy fromwireless battery charger 932 to the embedded MFS 902 in the mostefficient manner as is well known in the art.

In some embodiments, the microwave field stimulator 902 may be adaptedfor placement at the epidural layer of a spinal column, near or on thedura of the spinal column, in tissue in close proximity to the spinalcolumn, in tissue located near a dorsal horn, in dorsal root ganglia, inone or more of the dorsal roots, in dorsal column fibers, or inperipheral nerve bundles leaving the dorsal column of the spine.

In this embodiment, the microwave field stimulator 902 shall transmitpower and parameter signals to a passive Tx antenna also embeddedsubcutaneously, which shall be coupled to the RX antenna within thewireless stimulator device 922. The power required in this embodiment issubstantially lower since the TX antenna and the RX antenna are alreadyin body tissue and there is no requirement to transmit the signalthrough the skin.

FIG. 10 is a detailed diagram of an example microwave field stimulator902. A microwave field stimulator 902 may include a microcontroller 908,a telemetry feedback module 1002, and a power management module 1004.The microwave field stimulator 902 has a two-way communication schemawith a programmer 912, as well as with a communication or telemetryantenna 1006. The microwave field stimulator 902 sends output power anddata signals through a TX antenna 1008.

The microcontroller 908 may include a storage device 1014, a bluetoothinterface 1013, a USB interface 1012, a power interface 1011, ananalog-to-digital converter (ADC) 1016, and a digital to analogconverter (DAC) 1015. Implementations of a storage device 1014 mayinclude non-volatile memory, such as, for example, static electricallyerasable programmable read-only memory (SEEPROM) or NAND flash memory. Astorage device 1014 may store waveform parameter information for themicrocontroller 908 to synthesize the output signal used by modulator909. The stimulation waveform may include multiple pulses. The waveformparameter information may include the shape, duration, amplitude of eachpulse, as well as pulse repetition frequency. A storage device 1014 mayadditionally store polarity assignment information for each electrode ofthe wireless stimulator device 922. The Bluetooth interface 1013 and USBinterface 1012 respectively interact with either the bluetooth module1004 or the USB module to communicate with the programmer 912.

The communication antenna 1006 and a TX antenna 1008 may, for example,be configured in a variety of sizes and form factors, including, but notlimited to a patch antenna, a slot antenna, or a dipole antenna. The TXantenna 1008 may be adapted to transmit a transmission signal, inaddition to power, to the implantable, passive neural stimulator device922. As discussed above, an output signal generated by themicrocontroller 908 may be used by the modulator 909 to provide theinstructions for creation of a modulated RF carrier signal. The RFcarrier signal may be amplified by amplifier 906 to generate thetransmission signal. A directional coupler 1009 may be utilized toprovide two-way coupling so that both the forward power of thetransmission signal flow transmitted by the TX antenna 1008 and thereverse power of the reflected transmission may be picked up by powerdetector 1022 of telemetry feedback module 1002. In someimplementations, a separate communication antenna 1006 may function asthe receive antenna for receiving telemetry feedback signal from thewireless stimulator device 922. In some configurations, thecommunication antenna may operate at a higher frequency band than the TXantenna 1008. For example, the communication antenna 1006 may have acharacteristic frequency that is a second harmonic of the characteristicfrequency of TX antenna 1008, as discussed above.

In some embodiments, the microwave field stimulator 902 may additionallyinclude a telemetry feedback module 902. In some implementations, thetelemetry feedback module 1002 may be coupled directly to communicationantenna 1006 to receive telemetry feedback signals. The power detector1022 may provide a reading of both the forward power of the transmissionsignal and a reverse power of a portion of the transmission signal thatis reflected during transmission. The telemetry signal, forward powerreading, and reverse power reading may be amplified by low noiseamplifier (LNA) 1024 for further processing. For example, the telemetrymodule 902 may be configured to process the telemetry feedback signal bydemodulating the telemetry feedback signal to extract the encodedinformation. Such encoded information may include, for example, a statusof the wireless stimulator device 922 and one or more electricalparameters associated with a particular channel (electrode) of thewireless stimulator device 922. Based on the decoded information, thetelemetry feedback module 1002 may be used to calculate a desiredoperational characteristic for the wireless stimulator device 922.

Some embodiments of the MFS 902 may further include a power managementmodule 1004. A power management module 1004 may manage various powersources for the MFS 902. Example power sources include, but are notlimited to, lithium-ion or lithium polymer batteries. The powermanagement module 1004 may provide several operational modes to savebattery power. Example operation modes may include, but are not limitedto, a regular mode, a low power mode, a sleep mode, a deepsleep/hibernate mode, and an off mode. The regular mode providesregulation of the transmission of transmission signals and stimulus tothe wireless stimulator device 922. In regular mode, the telemetryfeedback signal is received and processed to monitor the stimuli asnormal. Low-power mode also provides regulation of the transmission oftransmission signals and stimulus to the electrodes of the wirelessstimulation device. However, under this mode, the telemetry feedbacksignal may be ignored. More specifically, the telemetry feedback signalencoding the stimulus power may be ignored, thereby saving MFS 902overall power consumption. Under sleep mode, the transceiver andamplifier 906 are turned off, while the microcontroller is kept on withthe last saved state in its memory. Under the deep sleep/hibernate mode,the transceiver and amplifier 906 are turned off, while themicrocontroller is in power down mode, but power regulators are on.Under the off mode, all transceiver, microcontroller and regulators areturned off achieving zero quiescent power.

FIG. 11 is a flowchart showing an example process in which the microwavefield stimulator 902 transmits polarity setting information to thewireless stimulator device 922. Polarity assignment information isstored in a non-volatile memory 1102 within the microcontroller 908 ofthe MFS 902. The polarity assignment information may berepresentative-specific and may be chosen to meet the specific need of aparticular patient. Based on the polarity assignment information chosenfor a particular patient, the microcontroller 908 executes a specificroutine for assigning polarity to each electrode of the electrode array.The particular patient has an wireless stimulation device as describedabove.

In some implementations, the polarity assignment procedure includessending a signal to the wireless stimulation device with an initialpower-on portion followed by a configuration portion that encodes thepolarity assignments. The power-on portion may, for example, simplyinclude the RF carrier signal. The initial power-on portion has aduration that is sufficient to power-on the wireless stimulation deviceand allow the device to reset into a configuration mode. Once in theconfiguration mode, the device reads the encoded information in theconfiguration portion and sets the polarity of the electrodes asindicated by the encoded information.

Thus, in some implementations, the microcontroller 908 turns on themodulator 909 so that the unmodulated RF carrier is sent to the wirelessstimulator device 1104. After a set duration, the microcontroller 908automatically initiates transmitting information encoding the polarityassignment. In this scenario, the microcontroller 908 transmits thepolarity settings in the absence of handshake signals from the wirelessstimulation device. Because the microwave field stimulator 902 isoperating in close proximity to wireless stimulator device 922, signaldegradation may not be severe enough to warrant the use of handshakesignals to improve quality of communication.

To transmit the polarity information, the microcontroller 908 reads thepolarity assignment information from the non-volatile memory andgenerates a digital signal encoding the polarity information 1106. Thedigital signal encoding the polarity information may be converted to ananalog signal, for example, by a digital-to-analog (DAC) converter 1112.The analog signal encoding the waveform may modulate a carrier signal atmodulator 909 to generate a configuration portion of the transmissionsignal (1114). This configuration portion of the transmission signal maybe amplified by the power amplifier 906 to generate the signal to betransmitted by antenna 907 (1116). Thereafter, the configuration portionof the transmission signal is transmitted to the wireless stimulatordevice 922 (1118).

Once the configuration portion is transmitted to the wirelessstimulation device, the microcontroller 908 initiates the stimulationportion of the transmission signal. Similar to the configurationportion, the microcontroller 908 generates a digital signal that encodesthe stimulation waveform. The digital signal is converted to an analogsignal using the DAC. The analog signal is then used to modulate acarrier signal at modulator 909 to generate a stimulation portion of thetransmission signal.

In other implementations, the microcontroller 908 initiates the polarityassignment protocol after the microcontroller 908 has recognized apower-on reset signal transmitted by the neural stimulator. The power-onreset signal may be extracted from a feedback signal received bymicrocontroller 908 from the wireless stimulator device 922. Thefeedback signal may also be known as a handshake signal in that italerts the microwave field stimulator 902 of the ready status of thewireless stimulator device 922. In an example, the feedback signal maybe demodulated and sampled to digital domain before the power-on resetsignal is extracted in the digital domain.

FIG. 12 is a flow chart showing an example of the process in which themicrowave field stimulator 902 receives and processes the telemetryfeedback signal to make adjustments to subsequent transmissions.

In some implementations, the microcontroller 908 polls the telemetryfeedback module 1002 (1212). The polling is to determine whether atelemetry feedback signal has been received (1214). The telemetryfeedback signal may include information based on which the MFS 902 mayascertain the power consumption being utilized by the electrodes of thewireless stimulator device 922. This information may also be used todetermine the operational characteristics of the combination system ofthe MFS 902 and the wireless stimulator device 922, as will be discussedin further detail in association with FIG. 13. The information may alsobe logged by the microwave field stimulator 902 so that the response ofthe patient may be correlated with past treatments received over time.The correlation may reveal the patient's individual response to thetreatments the patient has received up to date.

If the microcontroller 908 determines that telemetry feedback module1002 has not yet received telemetry feedback signal, microcontroller 908may continue polling (1212). If the microcontroller 908 determines thattelemetry feedback module 1002 has received telemetry feedback signal,the microcontroller 908 may extract the information contained in thetelemetry feedback signal to perform calculations (1216). The extractionmay be performed by demodulating the telemetry feedback signal andsampling the demodulated signal in the digital domain. The calculationsmay reveal operational characteristics of the wireless stimulator device922, including, for example, voltage or current levels associated with aparticular electrode, power consumption of a particular electrode,and/or impedance of the tissue being stimulated through the electrodes.

Thereafter, in certain embodiments, the microcontroller 908 may storeinformation extracted from the telemetry signals as well as thecalculation results (1218). The stored data may be provided to a userthrough the programmer upon request (1220). The user may be the patient,the doctor, or representatives from the manufacturer. The data may bestored in a non-volatile memory, such as, for example, NAND flash memoryor EEPROM.

In other embodiments, a power management schema may be triggered 1222 bythe microcontroller (908). Under the power management schema, themicrocontroller 908 may determine whether to adjust a parameter ofsubsequent transmissions (1224). The parameter may be amplitude or thestimulation waveform shape. In one implementation, the amplitude levelmay be adjusted based on a lookup table showing a relationship betweenthe amplitude level and a corresponding power applied to the tissuethrough the electrodes. In one implementation, the waveform shape may bepre-distorted to compensate for a frequency response of the microwavefield stimulator 902 and the wireless stimulator device 922. Theparameter may also be the carrier frequency of the transmission signal.For example, the carrier frequency of the transmission signal may bemodified to provide fine-tuning that improves transmission efficiency.

If an adjustment is made, the subsequently transmitted transmissionsignals are adjusted accordingly. If no adjustment is made, themicrocontroller 908 may proceed back to polling the telemetry feedbackmodule 1002 for telemetry feedback signal (1212).

In other implementations, instead of polling the telemetry feedbackmodule 1002, the microcontroller 908 may wait for an interrupt requestfrom telemetry feedback module 1002. The interrupt may be a softwareinterrupt, for example, through an exception handler of the applicationprogram. The interrupt may also be a hardware interrupt, for example, ahardware event and handled by an exception handler of the underlyingoperating system.

FIG. 13 is a schematic of an example implementation of the power, signaland control flow for the wireless stimulator device 922. A DC source1302 obtains energy from the transmission signal received at thewireless stimulator device 922 during the initial power-on portion ofthe transmission signal while the RF power is ramping up. In oneimplementation, a rectifier may rectify the received power-on portion togenerate the DC source 1302 and a capacitor 1304 may store a charge fromthe rectified signal during the initial portion. When the stored chargereaches a certain voltage (for example, one sufficient or close tosufficient to power operations of the wireless stimulator device 922),the power-on reset circuit 1306 may be triggered to send a power-onreset signal to reset components of the neural stimulator. The power-onset signal may be sent to circuit 1308 to reset, for example, digitalregisters, digital switches, digital logic, or other digital components,such as transmit and receive logic 1310. The digital components may alsobe associated with a control module 1312. For example, a control module1312 may include electrode control 252, register file 732, etc. Thepower-on reset may reset the digital logic so that the circuit 1308begins operating from a known, initial state.

In some implementations, the power-on reset signal may subsequentlycause the FPGA circuit 1308 to transmit a power-on reset telemetrysignal back to MFS 902 to indicate that the implantable wirelessstimulator device 922 is ready to receive the configuration portion ofthe transmission signal that contains the polarity assignmentinformation. For example, the control module 1312 may signal the RX/TXmodule 1310 to send the power-on reset telemetry signal to the telemetryantenna 1332 for transmission to MFS 902.

In other implementations, the power-on reset telemetry signal may not beprovided. As discussed above, due to the proximity between MFS 902 andimplantable, passive stimulator device 922, signal degradation due topropagation loss may not be severe enough to warrant implementations ofhandshake signals from the implantable, passive stimulator device 922 inresponse to the transmission signal. In addition, the operationalefficiency of implantable, passive neural stimulator device 922 may beanother factor that weighs against implementing handshake signals.

Once the FPGA circuit 1308 has been reset to an initial state, the FPGAcircuit 1308 transitions to a configuration mode configured to readpolarity assignments encoded on the received transmission signal duringthe configuration state. In some implementations, the configurationportion of the transmission signal may arrive at the wirelessstimulation device through the RX antenna 1334. The transmission signalreceived may provide an AC source 1314. The AC source 1314 may be at thecarrier frequency of the transmission signal, for example, from about300 MHz to about 8 GHz.

Thereafter, the control module 1312 may read the polarity assignmentinformation and set the polarity for each electrode through the analogmux control 1316 according to the polarity assignment information in theconfiguration portion of the received transmission signal. The electrodeinterface 252 may be one example of analog mux control 1316, which mayprovide a channel to a respective electrode of the implantable wirelessstimulator device 922.

Once the polarity for each electrode is set through the analog muxcontrol 1316, the implantable wireless stimulator device 922 is ready toreceive the stimulation waveforms. Some implementations may not employ ahandshake signal to indicate the wireless stimulator device 922 is readyto receive the stimulation waveforms. Rather, the transmission signalmay automatically transition from the configuration portion to thestimulation portion. In other implementations, the implantable wirelessstimulator device 922 may provide a handshake signal to inform the MFS902 that implantable wireless stimulator device 922 is ready to receivethe stimulation portion of the transmission signal. The handshakesignal, if implemented, may be provided by RX/TX module 1310 andtransmitted by telemetry antenna 1332.

In some implementations, the stimulation portion of the transmissionsignal may also arrive at implantable wireless stimulation devicethrough the RX antenna 1334. The transmission signal received mayprovide an AC source 1314. The AC source 1314 may be at the carrierfrequency of the transmission signal, for example, from about 300 MHz toabout 8 GHz. The stimulation portion may be rectified and conditioned inaccordance with discussions above to provide an extracted stimulationwaveform. The extracted stimulation waveform may be applied to eachelectrode of the implantable wireless stimulator device 922. In someembodiments, the application of the stimulation waveform may beconcurrent, i.e., applied to the electrodes all at once. As discussedabove, the polarity of each electrode has already been set and thestimulation waveform has been applied to the electrodes in accordancewith the polarity settings for the corresponding channel.

In some implementations, each channel of analog mux control 1316 isconnected to a corresponding electrode and may have a reference resistorplaced serially. For example, FIG. 13 shows reference resistors 1322,1324, 1326, and 1328 in a serial connection with a matching channel.Analog mux control 1316 may additionally include a calibration resistor1320 placed in a separate and grounded channel. The calibration resistor1320 is in parallel with a given electrode on a particular channel. Thereference resistors 1322, 1324, 1326, and 1328 as well as thecalibration resistor 1320 may also be known as sensing resistors 718.These resistors may sense an electrical parameter in a given channel, asdiscussed below.

In some configurations, an analog controlled carrier modulator mayreceive a differential voltage that is used to determine the carrierfrequency that should be generated. The generated carrier frequency maybe proportional to the differential voltage. An example analogcontrolled carrier modulator is VCO 733.

In one configuration, the carrier frequency may indicate an absolutevoltage, measured in terms of the relative difference from apre-determined and known voltage. For example, the differential voltagemay be the difference between a voltage across a reference resistorconnected to a channel under measurement and a standard voltage. Thedifferential voltage may be the difference between a voltage acrosscalibration resistor 1320 and the standard voltage. One example standardvoltage may be the ground.

In another configuration, the carrier frequency may reveal an impedancecharacteristic of a given channel. For example, the differential voltagemay be the difference between the voltage at the electrode connected tothe channel under measurement and a voltage across the referenceresistor in series. Because of the serial connection, a comparison ofthe voltage across the reference resistor and the voltage at theelectrode would indicate the impedance of the underlying tissue beingstimulated relative to the impedance of the reference resistor. As thereference resistor's impedance is known, the impedance of the underlyingtissue being stimulated may be inferred based on the resulting carrierfrequency.

For example, the differential voltage may be the difference between avoltage at the calibration resistor and a voltage across the referenceresistor. Because the calibration resistor is placed in parallel to agiven channel, the voltage at the calibration is substantially the sameas the voltage at the given channel. Because the reference resistor isin a serial connection with the given channel, the voltage at thereference resistor is a part of the voltage across the given channel.Thus, the difference between the voltage at the calibration resistor andthe voltage across the reference resistor correspond to the voltage dropat the electrode. Hence, the voltage at the electrode may be inferredbased on the voltage difference.

In yet another configuration, the carrier frequency may provide areading of a current. For example, if the voltage over referenceresistor 1322 has been measured, as discussed above, the current goingthrough reference resistor and the corresponding channel may be inferredby dividing the measured voltage by the impedance of reference resistor1322.

Many variations may exist in accordance with the specifically disclosedexamples above. The examples and their variations may sense one or moreelectrical parameters concurrently and may use the concurrently sensedelectrical parameters to drive an analog controlled modulator device.The resulting carrier frequency varies with the differential of theconcurrent measurements. The telemetry feedback signal may include asignal at the resulting carrier frequency.

The MFS 902 may determine the carrier frequency variation bydemodulating at a fixed frequency and measure phase shift accumulationcaused by the carrier frequency variation. Generally, a few cycles of RFwaves at the resulting carrier frequency may be sufficient to resolvethe underlying carrier frequency variation. The determined variation mayindicate an operation characteristic of the implantable wirelessstimulator device 922. The operation characteristics may include animpedance, a power, a voltage, a current, etc. The operationcharacteristics may be associated with an individual channel. Therefore,the sensing and carrier frequency modulation may be channel specific andapplied to one channel at a given time. Consequently, the telemetryfeedback signal may be time shared by the various channels of theimplantable wireless stimulator device 922.

In one configuration, the analog MUX 1318 may be used by the controllermodule 1312 to select a particular channel in a time-sharing scheme. Thesensed information for the particular channel, for example, in the formof a carrier frequency modulation, may be routed to RX/TX module 1310.Thereafter, RX/TX module 1310 transmits, through the telemetry antenna1332, to the MFS 902, the telemetry feedback encoding the sensedinformation for the particular channel.

Some implementations may include an application-specific integratedcircuit (ASIC) chip on the wireless stimulator device for processinginput signal and interfacing with the implanted electrodes. The ASICchip may be coupled to antenna(s) to receive the input signal from anexternal controller. The ASIC chip may harvest RF power from thereceived input signal to power the ASIC chip and the electrodes. TheASIC chip may also extract polarity setting information from thereceived input signal and use such information to set the polarity forelectrode interfaces. Moreover, the ASIC chip may extract waveformparameters from the received input signal and use such information tocreate electrical impulses for stimulating excitable tissues through theelectrodes. In particular, the ASIC chip may include a current steeringfeature to mirror currents to each electrode with evenness whilemaintaining a compact chip size.

FIG. 14 is a diagram of an example of ASIC chip 1400 for implantableuse. Chip 1400 may be fabricated based on a 0.6 um, double poly processutilizing High Value resistors, Schottky diodes and High VoltageTransistors. In some implementations, chip 1400 can be fabricated at awidth of 0.5 mm for fitting into, for example, an 18 Gauge needle. Inthese implementations, chip 1400 can have a length-width ratio of up to10 to 1. Chip 1400 can be coupled to, for example, either four (4) oreight (8) platinum-iridium electrodes that deliver electrical impulsesto tissue.

Chip 1400 includes RF to DC rectifying circuit 1402, a logic controlcircuit 1404, and a driving circuit 1406. RF to DC rectifying circuit1402 is coupled to differential antennas 1412A and 1412B. An RF inputsignal can be received at the differential antennas and then rectifiedto have the amplitude detected. The rectified signal may provide powerfor the chip 1400. Thereafter, logic control circuit 1404 may extractwaveform parameters from the amplitude detected signal. Subsequently,logic control circuit 1404 may generate electrical impulses according tothe extracted waveform parameters and solely based on the extractedelectric power. The generated electrical impulses may then be providedto the driving circuit 1406, which includes charge balancing and currentmirroring circuits. Driving circuit 1406 is coupled to electrodeinterfaces 1408A to 1408H, each coupled to a respective electrical load1409A through 1409H. The electrical impulses are subsequently deliveredto each electrode, namely 1410A through 1410H.

In this diagram, a diode bridge circuit 1414 is included to providefull-wave rectification to the input signal received in differentialform from differential antennas 1412A and 1412B. Full-wave rectificationmay utilize both the positive and negative portions of the RF inputsignal as received at differential antennas 1412A and 1412B.

In some implementations, a dipole antenna in a differentialconfiguration may be embedded into a wireless implantable stimulatordevice. The dipole antenna receives power, serial communication, andstimulus waveforms from an external transmitter placed outside thepatient's body. The dipole antenna is connected directly to a flexiblecircuit board embedded within the implantable stimulator device thatcontains discrete components and chip 1400. Chip 1400 can includewireless serial command receiver with up to eight channel multiplexingfunctionality.

The rectification may provide power to remaining portions of chip 1400.In some instances, VDD circuit 1418 and ground circuit 1419 are coupledto capacitor C1 1420 to provide stored charges. The stored charges maygenerally power chip 1400. In some implementations, a diode may be usedto supply the VDD logic supply from Vrect. If chip 1400 is active andthe voltage VDD dips below 1.8V, chip 1400 may enter into a “VDD lowvoltage recovery” mode. In this state any/all high side drivers will betemporarily over ridden to high impedance state (Hi-Z) and all low sidedrivers will be Hi-Z. Once VDD returns t6 above 3.0V state and in therunning mode the drivers would return to their previously programmedstate.

Output from rectifying circuit 1402 is coupled to the logic controlcircuit 1404. As depicted, logic control circuit 1404 may include logiccontrol/state machine 1422 and timer/oscillator 1424. Logiccontrol/state machine 1422 may be coupled to channel selector 1426.

The received RF input signal may contain waveform parameters forelectrical impulses to stimulate tissues. The received RF input signalmay contain polarity setting information for setting the interface foreach electrode.

Referring to FIG. 15, a sequence diagram during operation of the chip1400 is shown. Specifically, the pulse sequence 1500 includes segmentsof pulses. Each segment may last an epoch time (depicted as T_(epoch)).Each segment may include two portions, namely an initial portion and asubsequent stimulation portion. In more detail, the initial portionrefers to the portion in which electrical power contained in the RFinput signal is harvested and electrical charges are pumped intocapacitor C1 1420. The initial portion may last a period marked asThigh. The initial portion may be referred to as the communicationinitialization pulses 1502A and 1502B. The stimulation portioncorresponds to portion 1504 and may contain a serial message encodingwaveform parameters for electrical impulses and polarity settinginformation for the electrode interfaces. In some instances, portion1504 may be present in the first segment of sequence 1500 to configureelectrical impulses and polarity setting. Absent a power-on reset event,the configuration information of waveform parameters and polaritysetting may be fixed once the initialization is completed.

Chip 1400 may tolerate serial messages embedded between power bursts.For example, the transmitter may initiate a serial communication messageby sending a 2 ms “Communication Initialization Pulse”, CIP, (T_(high))followed by a 2 ms period of no power transmission (T_(msg)-T_(high)).In this example, data transmission may immediately follow this 2 msdelay and starts at time T_(msg). Bit timing calibration may beperformed by measuring the length of the header byte in the transmitteddata stream.

In some implementations, serial data may use a format based on the IrDASIR format. This coding format sends a pulse where the bit to be sent isa ‘0’. During bit times where the bit is set to “1,” no pulse may besent. Each pulse may be as short as 3/16ths of a bit time however thiswidth can be adjusted if necessary. This format may require less powerand therefore can allow serial data transmission to operate at lowerbaud rates.

In an example serial data communication, data can be transmittedasynchronously as bytes with 1 start and 1 stop bit (e.g. 8-n-1 formatcarries the same overhead to RS-232 with 10 bits transmitted but only 8of the 10 bits carry data while the other 2 bits are protocol overhead).The LSB may be the first data bit transmitted. This adds up to 70 bitstotal transmitted for 7 bytes of data. Ten of these bits are protocoloverhead and 60 of these bits are available to carry data. In thisexample, there is no additional delay between bytes, the data stream iscontinuous.

In the example, the serial baud rate is 19200. Serial messages can be ofa fixed length of 7 bytes, including a header byte, five payload bytes,and a checksum byte. Payload bytes may generally encode the polaritysetting for each electrode interface, the electrode drivers to use foreach electrode, the amplitude level for each electrode driver, etc. Thechecksum byte generally helps ensuring message integrity.

The header byte is used to identify the start of a data message. In someimplementations, it can be preset to the value OxAA. In theseimplementations, the header byte can be discarded until a correct headerbyte is received. The header may also be used to calibrate the internaloscillator, which is powered by wireless energy stored on VDD. Someimplementations may provide a unique structure ofon-off-on-off-on-off-on-off-on-off for the 10-bit sequence as timingmarkers at regular (104 μs between transmissions) intervals.

In some implementations, the header may include the address of theselected electrode array. For example, Bit 7 of Byte 1 can be the LeadAddress to distinguish between one of two possible electrode arrays isthe message intended. An electrode array may only implement messagesthat match its lead address assigned. If a lead of channel A receives amessage that is intended for channel B, the state machine may reject thenew message and maintain the previously stored register contents. Inthis example, each electrode array can have an address of 0 or 1 thatcan be determined by pin strapping during manufacture of the lead.

Returning to FIG. 14, in some implementations, AM detector 1416 mayoutput logic zero when RF power is received. In some implementations,pre-amplification of low voltage data signals or limiting of highvoltage data signals may extend the operational range of the AM detector1416. As such, signals 100 mV or greater will be detected. AM detector1416 may decode serial streams that are transmitted at 19200 Baud. TheAM detector 1416 input may be internal to chip 1400 and characterizedfor use at high frequencies (869-915 MHz).

AM detector 1416 may generally process rectified signals within anominal range from between 50 mVpp to 15 Vpp power supply levels (peakto peak). AM detector 1416 may include a preamp to clamp higher swingsignals without output collapsing or folding down. The preamp shouldhave sufficient gain and low offset to resolve 100 m Vpp data signals.

AM detector 1416 may detect serial data encoded using IrDA (SIR)formatting. The serial data receiver may be included in AM detector 1416and may convert the data from a serial format into a parallel format.Operations of the serial data receiver hardware may be controlled by aclock signal, which runs at a multiple of the data rate. In someimplementations, the receiver can test the state of the incoming signalon each clock pulse to search the start bit. If the apparent start bitis valid, then the bit signals the start of a new character. If not, thebit is considered a spurious pulse or power pulse and is ignored. Afterwaiting a further bit time, the state of the line is again sampled andthe resulting level clocked into a shift register.

After the required number of bit periods for the character length haveelapsed, the contents of the shift register are made available to thereceiving system. The serial data receiver has no shared timing systemwith the transmitter apart from the communication signal.

Serial data receiver on chip 1400 may receive and buffer seven (7)eight-bit words. The data contained in the words shall be used toprogram the control registers in the receiver IC LMI927 if a checksummatch is successful. The data will be ignored if a checksum match isunsuccessful and the receiver will continue to listen for valid data.The serial data receiver will reset and prepare to receive a new word ifa received byte does not meet IrDA (SIR) framing parameters. This willallow the serial receiver to quickly reset after being falsely activatedby reception of a spurious signal or a stim power pulse. The serial datareceiver will not have to wait to fully receive all words if anyindividual byte does not meet timing parameters.

Chip 1400 may remain in an un-configured state (all high-side outputsare high-Z, low-side outputs are in triode mode) until a valid set ofserial data is received. Notably, in some implementations, the serialreceiver may be not be operational if the Device Lock bit is set.

Logic control/state machine 1422 may be synchronized by timer/oscillator1424. The synchronization may enable logic control/state machine 1422 toextract and decode waveform parameters as well as polarity settinginformation from portion 1504. Logic control/state machine 1422 may thencreate one or more electrical impulses according to the waveformparameters. The Logic control/state machine 1422 may also set polaritiesof electrode interfaces 1408A to 1408H according to the extractedpolarity setting information.

The output of Logic Control/State Machine 1422 may be coupled to drivingcircuit 1406 which includes features of charge balancing, shuntresistors, and current mirroring. In particular, driving circuit 1406includes shunt resistor controller 1430 constructed to couple a shuntresistor to switch network 1432. The coupling can enhance defaultresistor 1431 through delay controller 1433. The delay controller mayinsert a corresponding shunt resistor to the circuit including thestimulating electrode at the end of an electrical impulse to reduce theamount of leakage current.

Some implementations may incorporate a variable shunt resistor tocontrol the discharge of the stimulus pulse from the DC-Blockingcapacitors. In these implementations, the initial serial commandscontain instructions for the set value for the shunt resistor. Forexample, the operator may select between four (4) different settings.The internal shunts are configured so that during a stimulus pulse theyare off, and after a pulse they are engaged.

The engagement of the resistors can be delayed following application ofthe electrical impulse. The timer is to delay the onset of the dischargeof the DC Blocking capacitors. The timer may be initialized during thestimulus pulse and it starts its delay at the end of the stimulus phase.The delay has a fixed duration and may be independent of the stimulusamplitude, repetition rate, and pulse width.

In some implementations, the stimulating electrical impulse is deliveredto a particular electrode through switch network 1432. To deliverelectrical impulses at both polarities, the switch network is coupled tocurrent source DAC 1434A and current sink DAC 1434B. As depicted,current source DAC 1434A includes a 7-bit dynamic range and is coupledto the rectifying voltage Vrect 1417. Current source DAC 1434A isinvoked with the polarity of the connected electrode set as positive.Similarly, current source DAC 1434A includes a 7-bit dynamic range.Current sink DAC 1434B is invoked with the polarity of the connectedelectrode set as negative. Current source DAC 1434A and current sink DAC1434B are complementary. Current source DAC 1434A and current sink DAC1434B both include current mirrors that can function to produce a copyof the current in one device, for example, the device that generatesvoltage Vrect 1417, by replicating the current in another device, forexample, current source DAC 1434A or current sink DAC 1434B. A currentmirror generally has a relatively high output resistance which helps tokeep the output current constant regardless of load conditions. Anotherfeature of the current mirror is a relatively low input resistance whichhelps to keep the input current constant regardless of drive conditions.

Chip 1400 may include a supervisory Power On Reset (POR) circuitdesigned to keep the device in reset until the system voltage hasreached the proper level and stabilized. The POR circuit also operatesas protection from brownout conditions when the supply voltage dropsbelow a minimum operating level. The POR circuit design is such that itincorporates appropriate hysteresis between reset and enable levels toprevent start up inrush currents from causing the device to reset duringnormal operating power-up conditions. The POR circuit performs as neededto maintain proper chip functionality under all power fluctuationconditions including high-speed transients and slow rate of changevoltage conditions. If required, the POR circuit can incorporate awatchdog timer tick event to ensure proper operation of the chip 1400.

As depicted in FIG. 14, each electrode interface is coupled to arespective capacitor 1409A through 1409H. These capacitors are placed inseries for the purpose of DC blocking. The capacitors are last in thesignal chain before the stimulation electrical impulses are delivered tothe electrodes. In some implementations, the nominal series capacitancemay be 3.0/μF at each electrode. Each capacitor in turn couples to arespective electrode 1410A through 1410H on an 8-electrode stimulatordevice. As noted, chip 1400 may be coupled to 8 electrode outputs. Eachelectrode output can be set to either sourcing, sinking or Hi-Z.

Referring to FIG. 16A, an example digital-to-analog mirror is shown forchip 1400. The example highlights a current mirroring feature.Generally, multiple DACs with individually addressable and controllablecurrent codes would increase ASIC's register space and design complexityand die area. On the other hand, having fewer DACs than availablechannels may require coulomb counting to limit the current throughindividual channels. With few DAC channels than the electrodes, thecurrent through individual electrodes may increase for lower impedancechannels.

In some implementations, a single current-steering DAC with ascaled-down least significant bit (LSB) current value is used togenerate a master bias current. In these implementations, no current maybe wasted in the current mirrors. After the master bias current isgenerated, the DAC current is then mirrored to individual electrodeswith a current mirror ratio of 1:N. Here, mirrored to individualelectrodes generally refers to connected via current mirrors toindividual electrodes. N can be selected based on current-steering DACmatching requirements. For example, N can be in the range of 10. The LSBsize of the individual electrodes may be 100 uA. With 7-bits, fulldynamic range of the driving current can be up to 12.7 mA.

The implementation depicted in FIG. 16A shows a dual-DAC approach withmirrored current sources across eight (8) electrodes. Transistor gate1603A is a replicate of transistor gate 1603B in the 1:1 transistormirror. Digital to Analog Conversion (DAC) circuit 1602A represents apush DAC and may correspond to a current sink. Meanwhile, DAC 1602Brepresents a pull DAC and may correspond to a current source. Logically,DAC 1602A and DAC 1602B may respectively correspond to DAC 1434A and DAC1434B as depicted in FIG. 14. To reduce current mirror mismatches andwasted current, an N-side, current-sink DAC is used for thisapplication. In this configuration, individual channels are enabled withcomplementary signals. Specifically, channel 0 is enabled bycomplementary transistors 1604A (for CH0 positive) and 1604B (for CH0negative). Transistor gate 1604A is coupled to Vrect 1417 whiletransistor gate 1604B is coupled to VDD 1418. Circuit 1605 representsthe tissue load on channel 0 as well as DC blocking capacitors. In someimplementations, the tissue load of channel 0 may include a capacitivecomponent in addition to the resistance component. Likewise,complementary transistors 1606A and 1606B respectively represent thepositive and negative polarity arrangements for driving tissue load 1607for channel 1. This implementation depicts an 8-side, current-sink DACconfiguration in which the current mirroring is replicated for eachchannel of the 8-channel electrode lead coupled to. For example, channel7 driving arrangements are represented by complementary transistors1608A and 1608B as well as tissue load 1608, as shown in FIG. 16A.Notably, in this N-side implementation, three states can be configuredfor each of the current sinks, namely, controlled (mirrored) currentsink mode, a cutoff device (off mode), and turned-on device as a triodemode switch. As depicted, each channel further includes an ON switch.For example channel 0 includes a CH0_ON switch, while channel 1 andchannel 7 respectively include CH1_ON and CH7_ON switches.

Based on this model, a variety of electrical signal parameters can bemodeled before an ASIC chip is fabricated. In one example, denotingstimulating waveform after rectification as Vstim, FIG. 16B shows theexpected Vstim waveform (on channel 1) and RF input signal (on channel2). As demonstrated, a great deal of detailed performance can besimulated during the design stage.

FIG. 17 provides an example timing of waveforms in the aboveimplementation. Trace 1712 shows the stimulation portion of a rectifiedRF input signal as seen on switch CH0_(N), while trace 1714A shows thewaveform seen on switch CH0 _(N) and trace 1714B shows the waveform seenon switch CH0_ON. The ON resistance of the triode mode may not becritical, since it is on during reverse discharge, and not duringstimulus mode.

Current steering as implemented (one current source and eight mirrors)may limit the charge per phase such that electrical impulses are appliedfor stimulation within safety ranges. In some implementations, theexternal transmitter may prescribe a limit on pulse width and theserial-written current level of the amplitude. With these parametersprescribed (or capped), a patient user is prevented from requesting anunsafe charge per phase because the patient user has limited parameterselection choices. In these implementations, when the stimulus portionis not present in the rectified RF input signal, the current DACs may beinactive.

To prevent a single electrode from sourcing or sinking more than theacceptable charge per phase, a current control approach can be used forboth high and low sides. In the current steering stimulus approach, highside is a single current source DAC connected to Vrect voltage withcurrent mirrors for each electrode. The low side is a current sink DAC.Each current steering DAC may include a 7-bit converter. Because a LSBcorresponds to 100 μA, the maximum current can be limited to 12.7 mA perelectrode. The master current reference for the DAC can be derived fromVrect. Following-current mirrors can be taken from Vrect. A similarapproach on the low side can be used to prevent a single electrode fromsinking too much charge.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. An integrated circuit for an implantablewirelessly powered device for implantation in a patient's body, thecircuit comprising: a radio-frequency (RF) to direct current (DC)rectifying circuit coupled to one or more antenna on the implantablewirelessly powered device, the rectifying circuit configured to: rectifyan input RF signal received at the one or more antennas and from anexternal controller through electric radiative coupling; and extract DCelectric power and configuration data from the input RF signal; a logiccontrol circuit connected to the rectifying circuit and a drivingcircuit, the logic control circuit configured to: generate a current forthe driving circuit solely using the extracted DC electrical power; inaccordance with the extracted configuration data, set polarity stateinformation for each electrode; and a driving circuit coupled to one ormore electrode, the driving circuit comprising current mirrors and beingconfigured to: steer, to each electrode and via the current mirrors, astimulating current solely from the generated current to modulate neuraltissue within the patient's body.